Skeleton documentation for the RISC-V port

During the upstreaming process it was suggested that I add a handful of
small documentation entries about the RISC-V port, which I've collected
here.

2018-01-29  Palmer Dabbelt  <palmer@sifive.com>

        * manual/math.texi: RISC-V supports _Float128 and _Float64x.

ChangeLog

@@ -2,6 +2,7 @@
 
 	* sysdeps/init_array/crti.S (.section .init_array): Add
 	PREINIT_FUNCTION when defined.
+	* manual/math.texi: RISC-V supports _Float128 and _Float64x.
 
 2018-01-29  Florian Weimer  <fweimer@redhat.com>
 

NEWS

@@ -41,13 +41,13 @@ Major new features:
   process aborts as the result of assertion failures.
 
 * On platforms where long double has the IEEE binary128 format (aarch64,
-  alpha, mips64, s390 and sparc), the math library now implements _Float128
-  interfaces for that type, as defined by ISO/IEC TS 18661-3:2015.  These
-  are the same interfaces added in version 2.26 for some platforms where
+  alpha, mips64, riscv, s390 and sparc), the math library now implements
+  _Float128 interfaces for that type, as defined by ISO/IEC TS 18661-3:2015.
+  These are the same interfaces added in version 2.26 for some platforms where
   this format is supported but is not the format of long double.
 
 * On platforms with support for _Float64x (aarch64, alpha, i386, ia64,
-  mips64, powerpc64le, s390, sparc and x86_64), the math library now
+  mips64, powerpc64le, riscv, s390, sparc and x86_64), the math library now
   implements interfaces for that type, as defined by ISO/IEC TS
   18661-3:2015.  These are corresponding interfaces to those supported for
   _Float128.
@@ -98,6 +98,14 @@ Major new features:
   to fail to load locales and fall back to the builtin C/POSIX locales.
   See notes below for other changes affecting compatibility.
 
+* Support for the RISC-V ISA running on Linux has been added.  This port
+  requires at least binutils-2.30, gcc-7.3.0, and linux-4.15; and is supported
+  for the following ISA and ABI pairs:
+
+    - rv64imac lp64
+    - rv64imafdc lp64
+    - rv64imafdc lp64d
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * Statically compiled applications attempting to load locales compiled for the

README

@@ -39,6 +39,7 @@ The GNU C Library supports these configurations for using Linux kernels:
 	powerpc64*-*-linux-gnu	Big-endian and little-endian.
 	s390-*-linux-gnu
 	s390x-*-linux-gnu
+	riscv64-*-linux-gnu
 	sh[34]-*-linux-gnu
 	sparc*-*-linux-gnu
 	sparc64*-*-linux-gnu

manual/math.texi

@@ -70,7 +70,7 @@ provided for @code{_Float32}, @code{_Float64} and @code{_Float32x} on
 all platforms.
 It is also provided for @code{_Float128} and @code{_Float64x} on
 powerpc64le (PowerPC 64-bits little-endian), x86_64, x86, ia64,
-aarch64, alpha, mips64, s390 and sparc.
+aarch64, alpha, mips64, riscv, s390 and sparc.
 
 @menu
 * Mathematical Constants::      Precise numeric values for often-used