libpayload: Add x86_64 (64-bit) support

This patch introduces x86_64 (64-bit) support to the payload, building
upon the existing x86 (32-bit) architecture. Files necessary for 64-bit
compilation are now guarded by the `CONFIG_LP_ARCH_X86_64` Kconfig
option.

BUG=b:242829490
TEST=Able to verify all valid combinations between coreboot and
payload with this patch.

Payload Entry Point Behavior with below code.

+----------------+--------------------+----------------------------+
| LP_ARCH_X86_64 | Payload Entry Mode | Description                |
+----------------+--------------------+----------------------------+
| No             | 32-bit             | Direct protected mode init |
+----------------+--------------------+----------------------------+
| Yes            | 32-bit             | Protected to long mode     |
+----------------+--------------------+----------------------------+
| Yes            | 64-bit             | Long mode initialization   |
+----------------+--------------------+----------------------------+

Change-Id: I69fda47bedf1a14807b1515c4aed6e3a1d5b8585
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81968
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/bin/lpgcc

@@ -84,7 +84,11 @@ if [ "$CONFIG_LP_ARCH_ARM64" = "y" ]; then
 fi
 if [ "$CONFIG_LP_ARCH_X86" = "y" ]; then
   _ARCHDIR=x86
-  _ARCHEXTRA="-m32 "
+  if [ "$CONFIG_LP_ARCH_X86_32" = "y" ]; then
+    _ARCHEXTRA="-m32 "
+  else
+    _ARCHEXTRA="-m64 "
+  fi
 fi
 if [ "$CONFIG_LP_ARCH_MOCK" = "y" ]; then
   _ARCHDIR=mock
@@ -122,13 +126,10 @@ CMDLINE=
 
 while [ $# -gt 0 ]; do
 	case $1 in
-	    -m32|-fno-stack-protector)
+	    -m32|-fno-stack-protector|-m64)
 		shift
 		continue
 		;;
-	    -m64)
-		error "Invalid option --64 - only 32 bit architectures are supported"
-		;;
 	    -c)
 		DOLINK=0
 		;;