Add RISC-V support

Upstream-Status: Submitted [https://bugzilla.mozilla.org/show_bug.cgi?id=1318905]

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>

--- a/build/autoconf/config.guess
+++ b/build/autoconf/config.guess
@@ -1029,6 +1029,9 @@ EOF
     ppcle:Linux:*:*)
 	echo powerpcle-unknown-linux-${LIBC}
 	exit ;;
+    riscv32:Linux:*:* | riscv64:Linux:*:*)
+	echo ${UNAME_MACHINE}-unknown-linux-${LIBC}
+	exit ;;
     s390:Linux:*:* | s390x:Linux:*:*)
 	echo ${UNAME_MACHINE}-ibm-linux-${LIBC}
 	exit ;;
--- a/build/moz.configure/init.configure
+++ b/build/moz.configure/init.configure
@@ -658,6 +658,9 @@ def split_triplet(triplet, allow_unknown
     elif cpu == 'sh4':
         canonical_cpu = 'sh4'
         endianness = 'little'
+    elif cpu in ('riscv32', 'riscv64'):
+        canonical_cpu = cpu
+        endianness = 'little'
     elif allow_unknown:
         canonical_cpu = cpu
         endianness = 'unknown'
--- a/python/mozbuild/mozbuild/configure/constants.py
+++ b/python/mozbuild/mozbuild/configure/constants.py
@@ -50,6 +50,8 @@ CPU_bitness = {
     'mips64': 64,
     'ppc': 32,
     'ppc64': 64,
+    'riscv32': 32,
+    'riscv64': 64,
     's390': 32,
     's390x': 64,
     'sh4': 32,
@@ -82,6 +84,8 @@ CPU_preprocessor_checks = OrderedDict((
     ('s390', '__s390__'),
     ('ppc64', '__powerpc64__'),
     ('ppc', '__powerpc__'),
+    ('riscv32', '__riscv && __SIZEOF_POINTER__ == 4'),
+    ('riscv64', '__riscv && __SIZEOF_POINTER__ == 8'),
     ('Alpha', '__alpha__'),
     ('hppa', '__hppa__'),
     ('sparc64', '__sparc__ && __arch64__'),