These are the packages that would be built, in order:

Calculating dependencies  ... done!
[ebuild   R    ] sys-apps/memtest86+-4.20-r1  USE="-floppy -serial" 0 kB

Total: 1 package (1 reinstall), Size of downloads: 0 kB

>>> Verifying ebuild manifests


>>> Emerging (1 of 1) sys-apps/memtest86+-4.20-r1
 * memtest86+-4.20.tar.gz SHA256 SHA512 WHIRLPOOL size ;-) ...           [ ok ]
>>> Unpacking source...
>>> Unpacking memtest86+-4.20.tar.gz to /var/tmp/portage/sys-apps/memtest86+-4.20-r1/work
>>> Source unpacked in /var/tmp/portage/sys-apps/memtest86+-4.20-r1/work
>>> Preparing source in /var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20 ...
 * Applying memtest86+-4.20-hardcoded_cc.patch ...
 [ ok ]
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20 ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20 ...
make -j1 
clang -fno-color-diagnostics -E -traditional head.S -o head.s
x86_64-pc-linux-gnu-as -32  -o head.o head.s
clang -fno-color-diagnostics -c -Wall -march=i486 -m32 -O2 -fomit-frame-pointer -fno-builtin -ffreestanding -fPIC -fno-stack-protector -fno-strict-aliasing -fno-stack-protector reloc.c
reloc.c:50:10: warning: variable 'got' is uninitialized when used here
      [-Wuninitialized]
        return *got;
                ^~~
reloc.c:49:26: note: initialize the variable 'got' to silence this warning
        register Elf32_Addr *got asm ("%ebx");
                                ^
                                 = NULL
1 warning generated.
clang -fno-color-diagnostics -Wall -march=i486 -m32 -O2 -fomit-frame-pointer -fno-builtin -ffreestanding -fPIC -fno-stack-protector -m32  -c -o main.o main.c
main.c:115:7: warning: passing 'volatile ulong *' (aka 'volatile unsigned long
      *') to parameter of type 'const void *' discards qualifiers
      [-Wincompatible-pointer-types-discards-qualifiers]
        goto *p;
             ^~
main.c:115:2: error: indirect goto in function with no address-of-label
      expressions
        goto *p;
        ^
1 warning and 1 error generated.
<builtin>: recipe for target 'main.o' failed
make: *** [main.o] Error 1
 * ERROR: sys-apps/memtest86+-4.20-r1::gentoo failed (compile phase):
 *   emake failed
 * 
 * If you need support, post the output of `emerge --info '=sys-apps/memtest86+-4.20-r1::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=sys-apps/memtest86+-4.20-r1::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/temp/environment'.
 * Working directory: '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20'
 * S: '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20'

>>> Failed to emerge sys-apps/memtest86+-4.20-r1, Log file:

>>>  '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/temp/build.log'
 * Messages for package sys-apps/memtest86+-4.20-r1:
 * ERROR: sys-apps/memtest86+-4.20-r1::gentoo failed (compile phase):
 *   emake failed
 * 
 * If you need support, post the output of `emerge --info '=sys-apps/memtest86+-4.20-r1::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=sys-apps/memtest86+-4.20-r1::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/temp/environment'.
 * Working directory: '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20'
 * S: '/var/tmp/portage/sys-apps/memtest86+-4.20-r1/work/memtest86+-4.20'