[vm] Fill in ELF attributes for RISC-V.

TEST=readelf
Change-Id: Ida1069f12998c8d12750d23843a7f964bf618077
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396100
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/platform/elf.h

@@ -168,6 +168,9 @@ static constexpr intptr_t EF_ARM_ABI_FLOAT_HARD = 0x00000400;
 static constexpr intptr_t EF_ARM_ABI_FLOAT_SOFT = 0x00000200;
 static constexpr intptr_t EF_ARM_ABI = 0x05000000;
 
+static constexpr intptr_t EF_RISCV_RVC = 0x1;
+static constexpr intptr_t EF_RISCV_FLOAT_ABI_DOUBLE = 0x4;
+
 static constexpr intptr_t EM_386 = 3;
 static constexpr intptr_t EM_ARM = 40;
 static constexpr intptr_t EM_X86_64 = 62;

runtime/vm/elf.cc

@@ -2120,6 +2120,8 @@ void ElfHeader::Write(ElfWriteStream* stream) const {
   uword flags = elf::EF_ARM_ABI | (TargetCPUFeatures::hardfp_supported()
                                        ? elf::EF_ARM_ABI_FLOAT_HARD
                                        : elf::EF_ARM_ABI_FLOAT_SOFT);
+#elif defined(TARGET_ARCH_RISCV32) || defined(TARGET_ARCH_RISCV64)
+  uword flags = elf::EF_RISCV_RVC | elf::EF_RISCV_FLOAT_ABI_DOUBLE;
 #else
   uword flags = 0;
 #endif