RISC-V: Tentative ".bfloat16" assembly support

This commit adds an assembler directive ".bfloat16" to help BFloat16
extensions ('Zfbfmin', 'Zvfbfmin' and 'Zvfbfwma') users.

gas/ChangeLog:

	* config/tc-riscv.c (FLT_CHARS): Add BFloat16 'b' to supported
	floating point formats.
	(riscv_pseudo_table) Add ".bfloat16" directive.
	* testsuite/gas/riscv/bfloat16.s: Copied from
	testsuite/gas/aarch64/bfloat16-directive.s.
	* testsuite/gas/riscv/bfloat16-be.d: New test ported from
	testsuite/gas/aarch64/bfloat16-directive-be.d and float16-be.d.
	* testsuite/gas/riscv/bfloat16-le.d: New test ported from
	testsuite/gas/aarch64/bfloat16-directive-le.d and float16-le.d.

Author: a4lg

gas/config/tc-riscv.c

@@ -439,7 +439,7 @@ const char EXP_CHARS[] = "eE";
 
 /* Chars that mean this number is a floating point constant.
    As in 0f12.456 or 0d1.2345e12.  */
-const char FLT_CHARS[] = "rRsSfFdDxXpPhH";
+const char FLT_CHARS[] = "rRsSfFdDxXpPhHb";
 
 /* Indicate we are already assemble any instructions or not.  */
 static bool start_assemble = false;
@@ -5164,6 +5164,7 @@ static const pseudo_typeS riscv_pseudo_table[] =
   {"attribute", s_riscv_attribute, 0},
   {"variant_cc", s_variant_cc, 0},
   {"float16", float_cons, 'h'},
+  {"bfloat16", float_cons, 'b'},
 
   { NULL, NULL, 0 },
 };

gas/testsuite/gas/riscv/bfloat16-be.d

@@ -0,0 +1,10 @@
+# source: bfloat16.s
+# objdump: -sj .data
+# as: -mbig-endian
+
+.*: +file format .*
+
+Contents of section \.data:
+ 0000 41403dfc 000042f7 8000c2f7 7fff7f80  .*
+ 0010 ff807f7f ff7f0080 80800001 8001007f  .*
+ 0020 807f3f80 bf804000 c000.*

gas/testsuite/gas/riscv/bfloat16-le.d

@@ -0,0 +1,10 @@
+# source: bfloat16.s
+# objdump: -sj .data
+# as: -mlittle-endian
+
+.*: +file format .*
+
+Contents of section \.data:
+ 0000 4041fc3d 0000f742 0080f7c2 ff7f807f  .*
+ 0010 80ff7f7f 7fff8000 80800100 01807f00  .*
+ 0020 7f80803f 80bf0040 00c0.*

gas/testsuite/gas/riscv/bfloat16.s

@@ -0,0 +1,19 @@
+.data
+	.bfloat16 12.0
+	.bfloat16 0.123
+	.bfloat16 +0.0
+	.bfloat16 123.4
+	.bfloat16 -0.0
+	.bfloat16 -123.4
+	.bfloat16 NaN
+	.bfloat16 Inf
+	.bfloat16 -Inf
+	.bfloat16 3.390e+38
+	.bfloat16 -3.390e+38
+	.bfloat16 1.175e-38
+	.bfloat16 -1.175e-38
+	.bfloat16 9.194e-41
+	.bfloat16 -9.194e-41
+	.bfloat16 1.167e-38
+	.bfloat16 -1.167e-38
+	.bfloat16 1.0, -1, 2.0, -2