std: Add compatibility with android-9

The Gecko folks currently use Android API level 9 for their builds, so they're
requesting that we move back our minimum supported API level from 18 to 9. Turns
out, ABI-wise at least, there's not that many changes we need to take care of.
The `ftruncate64` API appeared in android-12 and the `log2` and `log2f` APIs
appeared in android-18. We can have a simple shim for `ftruncate64` which falls
back on `ftruncate` and the `log2` function can be approximated with just
`ln(f) / ln(2)`.

This should at least get the standard library building on API level 9, although
the tests aren't quite happening there just yet. As we seem to be growing a
number of Android compatibility shims, they're now centralized in a common
`sys::android` module.

2 files changed, 10 insertions, 2 deletions

diff --git a/src/libstd/num/f32.rs b/src/libstd/num/f32.rs
index 624748f352e..94aa3d6b513 100644
--- a/src/libstd/num/f32.rs
+++ b/src/libstd/num/f32.rs
@@ -646,7 +646,10 @@ impl f32 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn log2(self) -> f32 {
-        unsafe { intrinsics::log2f32(self) }
+        #[cfg(target_os = "android")]
+        return ::sys::android::log2f32(self);
+        #[cfg(not(target_os = "android"))]
+        return unsafe { intrinsics::log2f32(self) };
     }
 
     /// Returns the base 10 logarithm of the number.
diff --git a/src/libstd/num/f64.rs b/src/libstd/num/f64.rs
index 6515301aefd..2beffb64d3d 100644
--- a/src/libstd/num/f64.rs
+++ b/src/libstd/num/f64.rs
@@ -546,7 +546,12 @@ impl f64 {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn log2(self) -> f64 {
-        self.log_wrapper(|n| { unsafe { intrinsics::log2f64(n) } })
+        self.log_wrapper(|n| {
+            #[cfg(target_os = "android")]
+            return ::sys::android::log2f64(n);
+            #[cfg(not(target_os = "android"))]
+            return unsafe { intrinsics::log2f64(n) };
+        })
     }
 
     /// Returns the base 10 logarithm of the number.