libbacktrace patch committed: Update README

libbacktrace patch committed: Update README

[Ian Lance Taylor]

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This patch updates the libbacktrace README to a near copy of the one
from github.com/ianlancetaylor/libbacktrace.  Committed to mainline.
This fixes GCC PR 105721.

Ian

* README: Update.

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6cf19361732bd7f8b41716ef9f4b5c205a3193b8
diff --git a/libbacktrace/README b/libbacktrace/README
index e8b225745c9..6225f92b855 100644
--- a/libbacktrace/README
+++ b/libbacktrace/README
@@ -1,23 +1,31 @@
 The libbacktrace library
-Initially written by Ian Lance Taylor <iant@google.com>
+Initially written by Ian Lance Taylor <iant@golang.org>
 
 The libbacktrace library may be linked into a program or library and
-used to produce symbolic backtraces.  Sample uses would be to print a
-detailed backtrace when an error occurs or to gather detailed
-profiling information.
+used to produce symbolic backtraces.
+Sample uses would be to print a detailed backtrace when an error
+occurs or to gather detailed profiling information.
+In general the functions provided by this library are async-signal-safe,
+meaning that they may be safely called from a signal handler.
 
-The libbacktrace library is provided under a BSD license.  See the
-source files for the exact license text.
+The libbacktrace library is provided under a BSD license.
+See the source files for the exact license text.
 
 The public functions are declared and documented in the header file
 backtrace.h, which should be #include'd by a user of the library.
 
 Building libbacktrace will generate a file backtrace-supported.h,
 which a user of the library may use to determine whether backtraces
-will work.  See the source file backtrace-supported.h.in for the
-macros that it defines.
+will work.
+See the source file backtrace-supported.h.in for the macros that it
+defines.
 
-As of September 2012, libbacktrace only supports ELF executables with
-DWARF debugging information.  The library is written to make it
-straightforward to add support for other object file and debugging
-formats.
+As of October 2020, libbacktrace supports ELF, PE/COFF, Mach-O, and
+XCOFF executables with DWARF debugging information.
+In other words, it supports GNU/Linux, *BSD, macOS, Windows, and AIX.
+The library is written to make it straightforward to add support for
+other object file and debugging formats.
+
+The library relies on the C++ unwind API defined at
+https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html
+This API is provided by GCC and clang.