util/xcompile: Allow overriding default compiler path

When looking for C compilers, xcompile uses the "" prefix to "gcc" and
"clang" as a last-resort option. This fails in environments where such
default names are blocked to prevent "unclean" builds - such as Chrome
OS.

Allow overriding this prefix using the GENERIC_COMPILER_PREFIX variable
that is hopefully both descriptive enough to suggest what it is for and
unusual enough to not trigger by chance.

Change-Id: I16239f66730f1dbcb7482f223cea4ee5957af10c
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56643
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

util/xcompile/xcompile

@@ -10,6 +10,8 @@ if [ "$1" = "--debug" ]; then
 	set -x
 fi
 
+# GENERIC_COMPILER_PREFIX defaults to empty but can be used to override
+# compiler search behavior
 TMPFILE=""
 XGCCPATH=$1
 
@@ -421,7 +423,7 @@ test_architecture() {
 
 	# Search toolchain by checking assembler capability.
 	for TBFDARCH in $TBFDARCHS; do
-		for gccprefix in $search ""; do
+		for gccprefix in $search "$GENERIC_COMPILER_PREFIX"; do
 			program_exists "${gccprefix}as" || continue
 			for endian in $TENDIAN ""; do
 				{ testas "$gccprefix" "$TWIDTH" "$TBFDARCH" \
@@ -439,7 +441,7 @@ test_architecture() {
 	fi
 
 	for clang_arch in $TCLIST invalid; do
-		for clang_prefix in $search $XGCCPATH ""; do
+		for clang_prefix in $search $XGCCPATH "$GENERIC_COMPILER_PREFIX"; do
 			testcc "${clang_prefix}clang" "-target ${clang_arch}-$TABI -c" && break 2
 		done
 	done