toolchain.inc: Set CCACHE_COMPILERCHECK=mtime

From ccache(1):

mtime
  Hash the compiler’s mtime and size, which is fast. This is the default.

Hashing the compiler binary's content would only be necessary when we
expect different binaries of the same size with the same path (e.g.
during a compiler bootstrap) and wrong modification timestamps (might
happen when checking an older version out of a (package) repository).
Neither should be the case during our builds. And even if everything
fails at once, chances are additionally low that a wrong cache hit
would cause a problem.

tl;dr we're building and testing firmware, not toolchains.

Change-Id: I264a72c628559384fcc2060d777c52af927d5e14
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76561
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

toolchain.inc

@@ -8,7 +8,7 @@ ifeq ($(CCACHE),)
 $(error ccache selected, but not found in PATH)
 endif
 
-export CCACHE_COMPILERCHECK=content
+export CCACHE_COMPILERCHECK=mtime
 export CCACHE_BASEDIR=$(top)
 export CCACHE_STATSLOG=$(obj)/ccache.stats