Add -mlong-double-64 option

gcc/config/riscv/riscv.h

@@ -115,7 +115,7 @@ along with GCC; see the file COPYING3.  If not see
 
 #define FLOAT_TYPE_SIZE 32
 #define DOUBLE_TYPE_SIZE 64
-#define LONG_DOUBLE_TYPE_SIZE 128
+#define LONG_DOUBLE_TYPE_SIZE (TARGET_LONG_DOUBLE_64 ? 64 : 128)
 
 /* Allocation boundary (in *bits*) for storing arguments in argument list.  */
 #define PARM_BOUNDARY BITS_PER_WORD

gcc/config/riscv/riscv.opt

@@ -88,6 +88,14 @@ mstrict-align
 Target Report Mask(STRICT_ALIGN) Save
 Do not generate unaligned memory accesses.
 
+mlong-double-64
+Target Report RejectNegative Mask(LONG_DOUBLE_64)
+Use 64-bit long double.
+
+mlong-double-128
+Target Report RejectNegative InverseMask(LONG_DOUBLE_64)
+Use 128-bit long double.
+
 Enum
 Name(code_model) Type(enum riscv_code_model)
 Known code models (for use with the -mcmodel= option):