MIPS64: Extend random address range in OS::GetRandomMmapAddr

MIPS64 is a 64-bit platform and each user space process can
address 2^42 bytes of memory. This fixes the current behavior
when the address range was limited to 2^30, which is the
default value taken for 32-bit platforms.

Change-Id: I310e16631a4dfaf77416e278ddf4386f3e258cc3
Reviewed-on: https://chromium-review.googlesource.com/c/1304197Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ibogosavljevic@wavecomp.com>
Cr-Commit-Position: refs/heads/master@{#57112}

src/base/platform/platform-posix.cc

@@ -237,6 +237,10 @@ void* OS::GetRandomMmapAddr() {
   // 31 bits of virtual addressing.  Truncate to 29 bits to allow kernel chance
   // to fulfill request.
   raw_addr &= 0x1FFFF000;
+#elif V8_TARGET_ARCH_MIPS64
+  // 42 bits of virtual addressing. Truncate to 40 bits to allow kernel chance
+  // to fulfill request.
+  raw_addr &= uint64_t{0xFFFFFF0000};
 #else
   raw_addr &= 0x3FFFF000;