[RISCV] Support -target-abi at the MC layer and for codegen

This patch adds proper handling of -target-abi, as accepted by llvm-mc and
llc. Lowering (codegen) for the hard-float ABIs will follow in a subsequent
patch. However, this patch does add MC layer support for the hard float and
RVE ABIs (emission of the appropriate ELF flags
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#-file-header).

ABI parsing must be shared between codegen and the MC layer, so we add
computeTargetABI to RISCVUtils. A warning will be printed if an invalid or
unrecognized ABI is given.

Differential Revision: https://reviews.llvm.org/D59023

llvm-svn: 355771
diff --git a/llvm/lib/Target/RISCV/Utils/RISCVBaseInfo.h b/llvm/lib/Target/RISCV/Utils/RISCVBaseInfo.h
index 49cae3a..4ace9dd 100644
--- a/llvm/lib/Target/RISCV/Utils/RISCVBaseInfo.h
+++ b/llvm/lib/Target/RISCV/Utils/RISCVBaseInfo.h
@@ -152,6 +152,26 @@
 #include "RISCVGenSystemOperands.inc"
 } // end namespace RISCVSysReg
 
+namespace RISCVABI {
+
+enum ABI {
+  ABI_ILP32,
+  ABI_ILP32F,
+  ABI_ILP32D,
+  ABI_ILP32E,
+  ABI_LP64,
+  ABI_LP64F,
+  ABI_LP64D,
+  ABI_Unknown
+};
+
+// Returns the target ABI, or else a StringError if the requested ABIName is
+// not supported for the given TT and FeatureBits combination.
+ABI computeTargetABI(const Triple &TT, FeatureBitset FeatureBits,
+                     StringRef ABIName);
+
+} // namespace RISCVABI
+
 } // namespace llvm
 
 #endif