Set GCC_TOOLCHAIN_DIR for Linux builds so they get the right 'as'.

Previously this was being inferred from the LD we passed, but this
was broken by the Linux Makefile change in
https://hafnium-review.googlesource.com/c/hafnium/third_party/linux/+/7943,
which resulted in the build using the system assembler from
/usr/bin/as instead, which may fail and is not hermetic.

Bug: 132428451
Change-Id: I3ff7bf18d7a68435b9b5a4e9ddd1f9dcbd8d93d3
1 file changed