[SPARK-30121][BUILD] Fix memory usage in sbt build script

### What changes were proposed in this pull request?
1. the default memory setting is missing in usage instructions
```
build/sbt -h
```
before
```
-mem    <integer>  set memory options (default: , which is -Xms2048m -Xmx2048m -XX:ReservedCodeCacheSize=256m)
```
after
```
-mem    <integer>  set memory options (default: 2048, which is -Xms2048m -Xmx2048m -XX:ReservedCodeCacheSize=256m)
```
2. the Perm space is not needed anymore, since java7 is removed.

the changes in this pr are based on the main sbt script of the newest stable version 1.3.4.

### Why are the changes needed?

bug fix

### Does this PR introduce any user-facing change?

no

### How was this patch tested?

manually

Closes apache#26757 from yaooqinn/SPARK-30121.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: yaooqinn

build/sbt

@@ -66,7 +66,7 @@ Usage: $script_name [options]
   -sbt-dir   <path>  path to global settings/plugins directory (default: ~/.sbt)
   -sbt-boot  <path>  path to shared boot directory (default: ~/.sbt/boot in 0.11 series)
   -ivy       <path>  path to local Ivy repository (default: ~/.ivy2)
-  -mem    <integer>  set memory options (default: $sbt_mem, which is $(get_mem_opts $sbt_mem))
+  -mem    <integer>  set memory options (default: $sbt_default_mem, which is $(get_mem_opts $sbt_default_mem))
   -no-share          use all local caches; no sharing
   -no-global         uses global caches, but does not use global ~/.sbt directory.
   -jvm-debug <port>  Turn on JVM debugging, open at the given port.

build/sbt-launch-lib.bash

@@ -17,6 +17,7 @@ declare -a java_args
 declare -a scalac_args
 declare -a sbt_commands
 declare -a maven_profiles
+declare sbt_default_mem=2048
 
 if test -x "$JAVA_HOME/bin/java"; then
     echo -e "Using $JAVA_HOME as default JAVA_HOME."
@@ -111,11 +112,10 @@ addDebugger () {
 # a ham-fisted attempt to move some memory settings in concert
 # so they need not be dicked around with individually.
 get_mem_opts () {
-  local mem=${1:-2048}
-  local perm=$(( $mem / 4 ))
-  (( $perm > 256 )) || perm=256
-  (( $perm < 4096 )) || perm=4096
-  local codecache=$(( $perm / 2 ))
+  local mem=${1:-$sbt_default_mem}
+  local codecache=$(( $mem / 8 ))
+  (( $codecache > 128 )) || codecache=128
+  (( $codecache < 2048 )) || codecache=2048
 
   echo "-Xms${mem}m -Xmx${mem}m -XX:ReservedCodeCacheSize=${codecache}m"
 }