elf: Add RISC-V PSABI ELF header defines

Refer to the RISC-V PSABI specification for details:

- https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

Cc: Michael Tokarev <mjt@tls.msk.ru>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

include/elf.h

@@ -1393,6 +1393,16 @@ typedef struct {
 #define R_RISCV_SET16         55
 #define R_RISCV_SET32         56
 
+/* RISC-V ELF Flags.  */
+#define EF_RISCV_RVC              0x0001
+#define EF_RISCV_FLOAT_ABI        0x0006
+#define EF_RISCV_FLOAT_ABI_SOFT   0x0000
+#define EF_RISCV_FLOAT_ABI_SINGLE 0x0002
+#define EF_RISCV_FLOAT_ABI_DOUBLE 0x0004
+#define EF_RISCV_FLOAT_ABI_QUAD   0x0006
+#define EF_RISCV_RVE              0x0008
+#define EF_RISCV_TSO              0x0010
+
 typedef struct elf32_rel {
   Elf32_Addr	r_offset;
   Elf32_Word	r_info;