Unbreak check-all on macOS after dbe8c2c3

`${X86_64}` expands to `x86_64;x86_64h` on macOS, so

    get_test_cc_for_arch(${X86_64} METADATA_TEST_TARGET_CC METADATA_TEST_TARGET_CFLAGS)

calls the macro get_test_cc_for_arch() with the four arguments
`x86_64`, `x86_64h`, `METADATA_TEST_TARGET_CC`, and `METADATA_TEST_TARGET_CFLAGS`.

This writes the compiler into a variable called x86_64h, the cflags into a
variable called METADATA_TEST_TARGET_CC, and silently ignores the fourth
parameter.

As a fix, just pass `x86_64` instead of `${X86_64}`. Hopefully
that won't break anything on other platforms.