tools/nolibc: add abs() and friends

This is used in various selftests and will be handy when integrating
those with nolibc.

Signed-off-by: Thomas Weißschuh <[e-mail]>
Acked-by: Willy Tarreau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Thomas Weißschuh <[e-mail]>

tools/include/nolibc/Makefile

···
 		fcntl.h \
 		getopt.h \
 		limits.h \
+		math.h \
 		nolibc.h \
 		poll.h \
 		signal.h \

tools/include/nolibc/math.h

···
+/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
+/*
+ * math definitions for NOLIBC
+ * Copyright (C) 2025 Thomas Weißschuh <[e-mail]>
+ */
+
+/* make sure to include all global symbols */
+#include "nolibc.h"
+
+#ifndef _NOLIBC_SYS_MATH_H
+#define _NOLIBC_SYS_MATH_H
+
+static __inline__
+double fabs(double x)
+{
+	return x >= 0 ? x : -x;
+}
+
+static __inline__
+float fabsf(float x)
+{
+	return x >= 0 ? x : -x;
+}
+
+static __inline__
+long double fabsl(long double x)
+{
+	return x >= 0 ? x : -x;
+}
+
+#endif /* _NOLIBC_SYS_MATH_H */

tools/include/nolibc/nolibc.h

···
 #include "fcntl.h"
 #include "getopt.h"
 #include "poll.h"
+#include "math.h"
 
 /* Used by programs to avoid std includes */
 #define NOLIBC

tools/include/nolibc/stdlib.h

···
  * As much as possible, please keep functions alphabetically sorted.
  */
 
+static __inline__
+int abs(int j)
+{
+	return j >= 0 ? j : -j;
+}
+
+static __inline__
+long labs(long j)
+{
+	return j >= 0 ? j : -j;
+}
+
+static __inline__
+long long llabs(long long j)
+{
+	return j >= 0 ? j : -j;
+}
+
 /* must be exported, as it's used by libgcc for various divide functions */
 void abort(void);
 __attribute__((weak,unused,noreturn,section(".text.nolibc_abort")))

tools/testing/selftests/nolibc/nolibc-test.c

···
 		CASE_TEST(tolower_noop);            EXPECT_EQ(1, tolower('a'), 'a'); break;
 		CASE_TEST(toupper);                 EXPECT_EQ(1, toupper('a'), 'A'); break;
 		CASE_TEST(toupper_noop);            EXPECT_EQ(1, toupper('A'), 'A'); break;
+		CASE_TEST(abs);                     EXPECT_EQ(1, abs(-10), 10); break;
+		CASE_TEST(abs_noop);                EXPECT_EQ(1, abs(10), 10); break;
 
 		case __LINE__:
 			return ret; /* must be last */