tree-wide: Replace exp10() with our own impl

exp10() has a symbol version > 2.34 on latest glibc. To allow 
dropping our baseline required glibc runtime version to <= 2.34, 
let's add our own version to prevent pulling in the newer symbol 
from glibc.

src/basic/math-util.c

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "math-util.h"
+
+double xexp10i(int n) {
+        /* Powers of 10 up to 10^22 are exact in IEEE-754 binary64. */
+        static const double table[] = {
+                1e0,  1e1,  1e2,  1e3,  1e4,  1e5,  1e6,  1e7,  1e8,  1e9,  1e10, 1e11,
+                1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19, 1e20, 1e21, 1e22,
+        };
+        bool negative = n < 0;
+
+        /* Cast before negation so n == INT_MIN doesn't invoke signed-overflow UB. Unsigned negation
+         * wraps to the magnitude we want. */
+        unsigned k = negative ? -(unsigned) n : (unsigned) n;
+
+        /* 10^309 already overflows binary64 to +Inf; anything beyond just stays there. */
+        k = MIN(k, 309u);
+        double r = k < ELEMENTSOF(table) ? table[k] : table[ELEMENTSOF(table) - 1];
+        for (unsigned i = ELEMENTSOF(table) - 1; i < k; i++)
+                r *= 10.0;
+
+        return negative ? 1.0 / r : r;
+}

src/basic/math-util.h

@@ -12,3 +12,8 @@
 /* To avoid x == y and triggering compile warning -Wfloat-equal. This returns false if one of the argument is
  * NaN or infinity. One of the argument must be a floating point. */
 #define fp_equal(x, y) iszero_safe((x) - (y))
+
+/* 10^n. Exact for |n| ≤ 22; otherwise multiplies and may accumulate rounding error. Saturates to
+ * 0.0 or +Inf outside binary64's exponent range; large |n| is capped internally so untrusted
+ * inputs can't cause unbounded work. */
+double xexp10i(int n);

src/basic/meson.build

@@ -64,6 +64,7 @@ basic_sources = files(
         'log.c',
         'log-context.c',
         'login-util.c',
+        'math-util.c',
         'memfd-util.c',
         'memory-util.c',
         'mempool.c',