gcc6 dependencies don't get installed

gcc6 depends on mpc1 and mpfr3 but these aren't getting installed.
Reproducing this issue:

$ pmbootstrap zap
$ pmbootstrap chroot -- apk add gcc6
$ pmbootstrap chroot -- /usr/gcc6/libexec/gcc/x86_64-alpine-linux-musl/6.4.0/cc1
Error loading shared library libmpc.so.3: No such file or directory (needed by /usr/gcc6/libexec/gcc/x86_64-alpine-linux-musl/6.4.0/cc1)
Error loading shared library libmpfr.so.4: No such file or directory (needed by /usr/gcc6/libexec/gcc/x86_64-alpine-linux-musl/6.4.0/cc1)
...

The package info has the related libraries listed as dependencies so:libmpc.so.3 and so:libmpfr.so.4.

gcc6-6.4.0-r9 depends on:
binutils
isl
so:gcc6:libstdc++.so.6
so:libc.musl-x86_64.so.1
so:libgcc_s.so.1
so:libgmp.so.10
so:libisl.so.15
so:libmpc.so.3
so:libmpfr.so.4
so:libz.so.1

The gcc package also has these two libraries and the two packages get installed properly.

gcc-8.3.0-r0 depends on:
binutils
isl
libgomp=8.3.0-r0
libatomic=8.3.0-r0
libgomp=8.3.0-r0
so:libc.musl-x86_64.so.1
so:libgcc_s.so.1
so:libgmp.so.10
so:libisl.so.15
so:libmpc.so.3
so:libmpfr.so.4
so:libstdc++.so.6
so:libz.so.1

I guess there hasn't been any issue building kernels with gcc6 because something else is causing mpc1 and mpfr3 to get installed when pmbootstrap builds the gcc6 kernels. I noticed this issue when using envkernel after zapping everything.

$ pmbootstrap zap
$ source ~/code/pmbootstrap/helpers/envkernel.sh --gcc6
$ make cyanogenmod_ouya_defconfig
*** pmbootstrap envkernel.sh active for /ubuntu_hdd/home/r/git/android_kernel_samsung_p4-ouya_downstream! ***
make: Entering directory '/mnt/linux'
  HOSTCC  scripts/basic/fixdep
Error loading shared library libmpc.so.3: No such file or directory (needed by /usr/gcc6/libexec/gcc/x86_64-alpine-linux-musl/6.4.0/cc1)
Error loading shared library libmpfr.so.4: No such file or directory (needed by /usr/gcc6/libexec/gcc/x86_64-alpine-linux-musl/6.4.0/cc1)
...