Refine r354661 to unbreak the GCC_BOOTSTRAP case.

[jhb]

Summary

MK_CLANG_IS_CC controls installing links for GCC, not just clang. Set
MK_CLANG_IS_CC to the value of MK_CLANG_BOOTSTRAP. This will leave it
as "no" if no bootstrap compiler is being built or GCC 4.2.1 is being
used as the bootstrap compiler, and "yes" if clang is being used as
the bootstrap compiler.

Test Plan

• Did 'make toolchain' for the following cases followed by 'make buildenv' and 'which cc' to verify that the correct 'cc' link was set. (Also, 'make toolchain' builds some things with the bootstrap compiler)
  • make TARGET_ARCH=amd64 (defaults to CLANG_BOOTSTRAP and CLANG_IS_CC=yes)
  • make TARGET_ARCH=mips (defaults to GCC_BOOTSTRAP and CLANG_IS_CC=no)
  • make TARGET_ARCH=riscv64sf WITH_CLANG_BOOTSTRAP (defaults to no bootstrap and CLANG_IS_CC=no and was the original case that broke)

Bryan had suggested setting MK_CLANG_IS_CC to MK_CLANG_BOOTSTRAP on
IRC earlier.